First of all, I believe that Richard Nixon already knew by 1968, or at the very latest in the early days of his presidency in 1969 that the Vietnam War was already lost. Just from his own knowledge about the war, his own advisers and from things that people around him have said. Now that doesn’t mean that you declare that the war is lost in public. He personally wasn’t ready to do that and it would’ve been bad policy as well. Because it would’ve send the message to South Vietnam to stop fighting. And to the North Vietnam to go stronger and see what that can get out of the South and from America.
So when Nixon comes to office in 1969, his strategy about Vietnam was exactly how the hell to get out of there and do it the best way possible. To continue to defend the South as best possible so the North couldn’t simply overwhelm those people and kill millions of innocent Vietnamese. While at the same time negotiate with the Communist North about how to end the war. And this went on forever. The war was officially over by late 1972 early 1973, but American troops and other government personal were still in Vietnam until early 1975 when we finally got out of there.